File:Twilight description full day.svg|thumb|300px|Twilight is the time between dawn and sunrise, or between sunset and dusk.
File:Top of Rock Cropped.jpg|thumb|300px|Midtown Manhattan at twilight, demonstrating blue hour.
'''Twilight''' is the illumination of the Earth's lower atmosphere when the Sun itself is not directly visible because it is below the horizon
. Twilight is produced by sunlight scattering
in the upper Earth's atmosphere|atmosphere
, illuminating the lower atmosphere so that the surface of the Earth
is neither completely lit nor completely dark. The word ''twilight'' is also used to denote the periods of time when this illumination occurs.
The further the Sun is below the horizon, the dimmer the twilight (other things such as atmospheric conditions being equal). When the Sun reaches 18 degrees below the horizon, the twilight's brightness is nearly...